The invention relates to a sealing unit for applying a plastic foil to a planar substrate material by means of heat and pressure using a sealing plate with an active surface and a back plate which is movable relative thereto, in which the sealing unit is linked to a die cutting unit with a die cutting knife and a counter plate for subsequently making die cuts at selected locations in the planar substrate material in the region of the applied plastic film.
Sealing units of the type defined above are known in the packaging manufacturing art. An example is a filling machine for forming and filling a liquid packaging, which is continuously manufactured from a tube of packaging material, with filling and sealing. In this known example, the packaging material is a planar carrier material coated with plastic, the carrier material being paper or board. Opening devices for said known packaging consist of a hole through the planar substrate material, wherein the hole is closed on the inner side of the packaging with a plastic film and on the outer side with a tear tab. The tear tabs and plastic foil are heat sealed together so that on tearing the tear tab to open the packaging, the region inside the punched out hole of the inner plastic foil is torn out as well, forming a dispensing opening.
Previously, the tear tab was directly heat sealed to the outer plastic foil of the packaging over the hole. However, this suffers from the disadvantage that on tearing the tear tab, the outer plastic foil is not torn out cleanly with it, so that frayed naked board and plastic are sometimes visible along with the coated printed board, which produces a poor mouth feel when the packaging is placed in the mouth to take a drink.
For that reason, a further plastic film was heat sealed in the region of the hole which is subsequently made, with film around the hole for sealing with the outer plastic layer of the packaging. That plastic film or plastic foil, known as a prepatch, is an oriented polypropylene (OPP) foil, which is transparent and can readily be applied to the plastic layer on the carrier material on the underside of the packaging and binds well therewith, although externally it only develops a peelable weak bond with the tear tab, particularly if the tear tab is polyethylene (PE) coated aluminium. The planar substrate material is cut through together with the applied plastic or prepatch foil formed from OPP film. If the tear tab is subsequently removed to open the packaging, then advantageously no more frayed small regions of naked board, coated board and plastic swarf are seen. Furthermore, applying a prepatch plastic foil, plastic prepatch or cut out provides the consumer with a good mouth feel when liquid is drunk from the packaging.
When manufacturing such packaging with that type of planar substrate material, a sealing unit of the aforementioned type is used to seal the prepatches or plastic foil to the face of the substrate material which will subsequently form the outside, and subsequently the dispensing hole is formed by die cutting entirely through the substrate material, as described above. Thus, a die cutting unit is linked to the sealing unit, in which a suitably shaped die cutting knife is punched through the whole substrate material with the various plastic layers and foils.
With such a die cutting procedure, however, the prior art encounters a problem in that the substrate material in the sealing unit for applying the plastic foil is heated to the sealing temperature and shortly thereafter the location for the die cut is fed into the die cutting unit, so the die cutting knife has to punch through substrate material which is still almost at the sealing temperature. Unfortunately, the cut lines produced by the die cutting knife become frayed to a greater or lesser extent and occasionally plastic or fibres or swarf are left hanging. If poorly cut openings are sealed over with a tear tab, then occasionally, the seal is not complete and that pack cannot be used as it leaks. With a large secondary packaging, just two or three unsealed packs are enough to make the whole secondary packaging unusable and unsaleable. Thus, strenuous efforts have been made to avoid such swarf near the die cutting knife. Thus, for example, it has been shown that a very sharp new die cutting knife with a circular knife could produce clean cuts for a certain period. After 150 hours of operation, however, the die cutting knife was so blunt and worn that the swarf occurred again. More disadvantageously, detritus collects in the outlet of the machine, resulting in blockages. Frequently changing the die cutting tools has the additional disadvantage of intolerable cost increases for the manufacturer.
Thus, the aim of the invention is to improve a sealing unit of the type defined above so that the cut edges no longer have swarf of carrier material or plastic but are clean, with no projections.